The invention relates to an apparatus used for a safety disconnection of an electrical load from an electrical power supply, for example a feed battery. The load may be an electrical load such as an electric motor with a high inductance. Safety disconnections or emergency disconnections are necessary when the electrical load must be forced to stop operating, for example when a fault occurs. Such safety disconnections serve, for example, to protect personnel against undesirable, uncontrolled and in some circumstances even dangerous actions of, for example, an electric motor load. Since relays are generally used for an emergency disconnection, these relays must operate in a fail-safe manner.
In the past, special safety relays have been used for safely disconnecting electrical loads. These safety relays have several parallel contact sets, which are driven in a mechanically forced manner. While one of the contact sets is used to pass on or interrupt the actual load current, the other contact set may have a test current applied to it. This test current can be evaluated to determine whether the contact sets, which are driven in a forced manner, have assumed a desired or undesired switching state, that is to say whether the safety relay is operating correctly or whether it is defective. However, this type of mechanical checking of relays with the aid of redundant contacts is complex.
It is accordingly an object of the invention to provide an apparatus for disconnecting an electrical load from a supply voltage which overcomes the above-mentioned disadvantages of the heretofore-known apparatuses of this general type and which does not require the use of special safety relays.
With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for disconnecting an electrical load from an input DC voltage of a DC voltage supply, including:
a first line carrying a given potential;
a second line carrying a reference potential;
the first line having an input connection to be connected to the DC voltage supply and having an output connection to be connected to the electrical load;
the first and second lines carrying the input DC voltage from the input connection to the output connection;
a fuse connected in series in the first line and being provided adjacent to the input connection, the fuse having a side opposite the input connection;
a first relay having a first switching contact and having a side opposite the input connection;
the first switching contact being connected in series in the first line on the side of the fuse opposite the input connection, the first switching contact being closed during a normal operation, and the first switching contact being opened for disconnecting the first line when a disconnection is initiated;
a second relay having a second switching contact; and
the second switching contact being connected in parallel between the first line and the second line on the side of the first switching contact opposite the input connection, the second switching contact being open during the normal operation, and being closed when the disconnection is initiated for short-circuiting the first line to the second line subsequent to the first switching contact being opened.
In other words, the circuit according to the invention is based on the safety of the disconnection process being achieved with the aid of a so-called xe2x80x9cchecked redundancyxe2x80x9d of staggered conventional relays. The configuration of the disconnection apparatus according to the invention has the particular advantage that a safe disconnection is achieved on the principle of the checked redundancy and diversity. In this way, it is possible to dispense with the use of special safety relays. Instead of using special safety relays, simple relays, for example mass-produced relays from a large-scale production for motor vehicles, can be used for the relays K1, K2, K3 which each have only one set of switching contacts. The invention has the advantage that a safety disconnection apparatus can be constructed using low-cost relays which, until now, could not be used in conventional safety circuits.
In accordance with another feature of the invention, a third relay having a third switching contact is provided. The third switching contact is connected in series in the first line on a side of the second switching contact opposite the input connection. The third switching contact is closed during the normal operation, and is opened when the disconnection is initiated for disconnecting the first line after the second switching contact is closed.
In accordance with yet another feature of the invention, the first, second, and third relays are commercially available relays, which have a single contact set.
In accordance with a further feature of the invention, the second line is a ground potential line.
In accordance with another feature of the invention, the first and second relays perform a high-availability disconnection.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an apparatus for a safe disconnection of an electrical load from an electrical DC voltage supply, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.